In a field of magnetic recording using a head and a medium, further improvement of performance of a magnetic recording medium and a magnetic recording head is in demand in association with the high recording density of a magnetic disk device.
A magnetic recording medium is a discontinuous medium where magnetic grains aggregate, and where each magnetic grain has a single magnetic domain structure. In such a magnetic recording medium, one recording bit is configured by a plurality of magnetic grains. Consequently, in order to enhance the recording density, by making the size of the magnetic grains smaller, the unevenness of the boundaries of adjacent recording bits has to be reduced. However, if the size of the magnetic nanoparticles is reduced, there is the problem that the thermal stability of magnetization of the magnetic grains is decreased.
As a countermeasure against this problem, although an increase in magnetic anisotropic energy Ku of the magnetic grains can be considered, the increase of Ku brings an increase in an anisotropic field (coercive force) of a magnetic recording medium. In the meantime, the upper limit of the recording magnetic field intensity by a magnetic recording head is substantially determined by the saturation magnetic flux density of a soft magnetic material that configures a magnetic core within a head. Consequently, if an anisotropic field of the magnetic recording medium exceeds the tolerance value determined from the upper limit of the recording magnetic field intensity, it is impossible to record into the magnetic recording medium.
At present, as one method for resolution of such thermal stability issue, a magnetic recording medium made of a magnetic material with great Ku is used. In the meantime, energy assisted recording that reduces effective recording magnetic field intensity by supplementally providing energy to the medium at the time of recording is proposed. The recording method where a microwave magnetic field is used as the supplemental energy source is referred to as microwave assisted magnetic recording (MAMR), for which research and development for practical use are in progress.
In microwave assisted magnetic recording, because a microwave magnetic field in a medium in-plane direction of a frequency according to an effective magnetic field (Heff) to be applied to magnetization of a recording layer in a magnetic recording medium is applied, precession movement of the magnetization in the recording layer is excited, and recording capability by a magnetic recording head is assisted.
As one example of a magnetic recording head where the microwave assisted magnetic recording method is adopted, as shown in FIG. 15, a magnetic recording head that is provided with a main magnetic pole 6′ that generates a recording magnetic field for applying to a magnetic recording medium, a wraparound shield which has a trailing shield 81′ and side shields 82′ and 83′, and a spin torque oscillator (STO) 10′ which has a multilayer structure of a magnetic thin film placed in a write gap between the main magnetic pole 6′ and the side shields 82′ and 83′, is proposed (for example, U.S. Pat. No. 9,047,887). The spin torque oscillator 10′ is an element where its magnetization fluctuates while precessing in response to the spin transfer torque, and its recording performance can be improved by having a mutual influence on the recording magnetic field (strengthening or weakening the recording magnetic field) by the magnetic field that is generated from the spin torque oscillator 10′. For example, the spin torque oscillator 10′ can generate a microwave magnetic field in an in-plane direction due to its oscillation. The precession movement of the magnetization in the recording layer is induced by superimposing the microwave magnetic field and the recording magnetic field on the magnetic recording medium, and perpendicular magnetization in the recording layer is reversed.
In such a magnetic recording head, in order that the magnetic field that is sufficiently generated from the spin torque oscillator 10′ has a mutual influence on the recording magnetic field, it is necessary to increase the spin transfer torque that acts on the spin torque oscillator 10′ and to increase variations of magnetization. However, in the magnetic recording head shown in FIG. 15, there is the problem that the oscillation of the spin torque oscillator 10′ is inhibited due to the magnetic coupling between the spin torque oscillator 10′ and the side shields 82′ and 83′. Further, since the magnetization of the side shields 82′ and 83′ fluctuates due to the magnetic coupling between the spin torque oscillator 10′ and the side shields 82′ and 83′, there is also concern that deterioration of writing (writing into an adjacent track) may occur (ATE: adjacent track erasure; WATE: wide area track erasure).